Certain acoustic and vibration attenuation techniques have focused on "add-on" viscoelastic damping treatments.
Add-on viscoelastic damping treatments as specified in MIL-STD-2148(SH) and MIL-P-23653C can add substantial weight to machinery foundations, bulkheads and other ship structures. For relatively light steel structures (i.e. less than 3/4 thick), there is enough vibration induced strain in a free layer viscoelastic add-on treatment to dissipate vibration effectively. For heavier structures the viscoelastic must be stressed in shear to get enough induced strain to dissipate vibration energy effectively. Thus for considerations of weight, a light weight, high bending stiffness constraining layer is required in a shear damping treatment.
Weight is not the only consideration in developing a successful add-on damping treatment. Material and installation cost, fire performance, toxicity, etc. are important additional constraints on the selection of an operational system.
Type VI Class 1 and Class 2 damping tiles are intended as light-weight damping substitutes for Type II Class 1 and Class 2 damping. The Type VI Class 1 damping tile having a constraining layer and a substrate was developed to replace the Type II Class 1 damping tile in an effort to achieve weight savings.
Currently, Type VI damping is specified as either a single layer of viscoelastic material or as a layer of viscoelastic material with a reinforced laminate cover bonded to it. The laminate cover is bonded to the viscoelastic material at the factory.
In one application, a somewhat light weight shear damping treatment (designated Type VI) was developed for submarines. The treatment consisted of using pre-fabricated Type VI damping tiles pre-configured to match the surface of the substrate to be damped. In such application, a viscoelastic material integrally is bonded to a pre-configured composite constraining layer consisting of Kevlar and graphite fiber materials in a bismaleimide (BMI) high-temperature-cure resin matrix (hereinafter "BMI Type VI damping tiles").
However, the aforementioned BMI Type VI damping tiles have inherent disadvantages which detract from their being accepted as an adequate substitute for Type II tiles. The BMI Type VI damping tiles are too costly from both a material and installation standpoint. The bismaleimide (BMI) resin used in the constraining layer is relatively expensive ($20-30/lb). In addition, the BMI Type VI damping tiles cannot be conformed-in-place during installation. These tiles are rigid and do not readily conform to curved surfaces.
In practice, BMI Type VI damping tiles must be procured in the form of pre-made damping tiles with specific contour and curvature and this substantially increases their cost. Effective damping performance requires the damping material and the structure, i.e. substrate, being damped to be in intimate contact with each other. On curved structures this is accomplished with Type II tiles by warming them slightly so they become pliable and then pressing them onto the surfaces to be damped. However, the stiff cured laminate cover of the pre-fabricated BMI Type VI damping tiles precludes this installation process. Therefore, whenever such BMI Type VI damping tiles are installed on contoured surfaces, such tiles must be procured with a matching curvature. Alternately, the flat BMI Type VI damping tiles can be cut into smaller pieces and then the pieces can be arranged, like a jig saw puzzle, onto the contoured surfaces. Alternatively, in the conventional BMI Type VI damping tile, the BMI-containing constraining layer can be pre-molded to a pre-determined curvature during the curing procedure. However, this is expensive since such molding must take place at high temperatures and high pressures in an autoclave. Still further, the BMI Type VI reinforced laminate cover, i.e. constraining layer, is difficult to cut and requires the use of a specialized band saw or water jet cutting tool. The cut edges must also be resealed with a resin to prevent moisture from wicking along the fiber structure of the laminate, particularly those based on Kevlar. This difficulty in cutting the laminate cover coupled with the need to reseal to cut edge further increases installation costs of BMI Type VI damping tiles.
In short, a Type VI damping tile which is relatively facilely installed, is cheaper to produce, and has effective acoustic and vibration damping properties has been sought in order have a truly cost-effective operational system. The present invention satisfies that need.